Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Dice the chicken breast into uniform 1-inch cubes to ensure even cooking throughout.
Chop the broccoli into small florets, dice the red bell pepper into squares, and thinly slice the red onion.
Place the chicken and vegetables directly on the prepared sheet pan, then drizzle with the extra virgin olive oil.
Evenly sprinkle the garlic powder, dried oregano, sea salt, and black pepper over the ingredients and toss thoroughly to coat.
Spread the mixture into a single layer across the pan, ensuring the chicken pieces are not crowded for maximum browning.
Roast in the center of the oven for 18-20 minutes, or until the chicken is cooked through and the vegetables are tender-crisp.
While the pan is roasting, gently warm the pre-cooked quinoa in a small saucepan or microwave.
Remove the pan from the oven, drizzle the fresh lemon juice over the top, and serve the chicken and vegetables over the warm quinoa.